The present invention relates to a pneumatic tire, more particularly to a motorcycle tire having a unidirectional tread pattern by which transient characteristics when a motorcycle is leaned to make a turn, can be improved without sacrificing the traction performance and tire life.
In the case of a motorcycle, when a rider initiates a turn and leans into the turn, the motorcycle rotates around its roll axis z (shown in FIG. 5) extending forward and obliquely upward from the ground contacting patch (pr) of the rear tire (ar), and a steering angle occurs on the front tire (af) according to the roll, thereby the motorcycle can make a turn.
In order to improve the cornering performance, the motorcycle tire is required to have easiness of leaning the motorcycle when enter the initial stage of the leaning from the vertical state during straight running. And in order to stop the leaning of the motorcycle after that, the motorcycle tire is required to have stability of leaning from the middle stage to the final stage of the leaning (full lean state). Further, the motorcycle tire is required to give the rider a linear transitional response from the initial stage to the final stage of the leaning. Thus, in the case of the motorcycle tires, it is required to have improved transient characteristics when the motorcycle is leaned to make a turn (hereinafter, the “roll characteristic”).
In the united states Patent Application Publication No. US 2009/308515 A1, a motorcycle tire reduced in the rolling resistance without deteriorating the above-mentioned roll characteristic is disclosed, wherein the tread portion is, as shown in FIG. 6, provided in the central ground contacting zone (a) during straight running with three circumferential grooves g1 and g2. As a result, the tire has easiness of leaning when the motorcycle enters the initial stage of the leaning from the vertical state during straight running.
However, the increase in the response about the leaning when the ground contacting area shifts from the central ground contacting zone (a) to the lateral ground contacting zone (b), becomes insufficient instead. As a result, from the middle stage to the final stage of the leaning, there is a tendency that the motorcycle rotates at a higher rate around its roll axis, and the cornering performance is deteriorated. Further, there is a tendency that the traction performance and tire life are deteriorated because the circumferential grooves g1 and g2 are disposed in the central zone (a) where the ground pressure becomes highest.